During the past year we have continued our studies on factors affecting the risk for cholesterol gallstones. First, we have developed a method for quantitative estimation of hepatic secretion in the fasting state. Since supersaturation of bile occurs mainly during fasting when bile is entering the gallbladder, it would seem important to be able to study the interrelations of biliary lipids during fasting. Second, we examined the role of several factors on bile saturation. One plasma cholesterol-lowering agent, nicotininc acid, was found to have little or no effect on bile, in contrast to clofibrate which causes marked supersaturation. The antibiotic, rifampin, causes striking supersaturation of bile and may increase risk for gallstones. Finally, we demonstrated that formation of cholesterol crystals in bile appears to be crucial for development of cholesterol gallstones. Many people have supersaturated bile but appear to be protected from gallstones because they do not develop crystals. Other develop crystals and usually get gallstones. We consider this latter finding to have considerable significance for our understanding of gallstone formation.